The present invention relates to a method and a machine for treating containers made of plastic such as bottles made of polyethylene terephthalate (PET).
The present invention relates more particularly to a method for treating at least one container aimed at depositing an internal coating forming a barrier by means of a microwave plasma, in particular with a view to enabling oxido-sensitive liquids to be packaged in the container, of the type in which the container is disposed inside a hermetically sealed treatment chamber which delimits a cavity outside the container and which is connected to a vacuum pumping circuit by an external vacuum line, the interior of the container being connected to the pumping circuit by an internal vacuum line, of the type comprising a preliminary step during which the pumping circuit produces a pressure reduction inside the cavity to a set value called the final external value, and a pressure reduction in the container to a set value called the final internal value, the preliminary step being followed by a treatment step during which the final values are maintained inside the cavity and inside the container so as to enable the internal coating to be deposited inside the container.
During the treatment step, a precursor fluid (for example based on acetylene in the case of the production of a carbon-containing coating, or an organosilicon-containing compound in the case of a silica-based deposit) is injected into the container and subjected to the action of microwaves so that it passes to the plasma state and produces a barrier deposit on the internal walls of the container.
In order to produce this deposit, it is necessary to create the vacuum inside the container and inside the cavity, and to maintain this vacuum during the entire treatment step.
Generally, the desired pressure inside the container is approximately 0.1 mbar and the desired pressure inside the cavity is approximately 50 mbar.
It is known to carry out the preliminary step by simultaneously pumping from the cavity and from the container until the pressure inside the cavity reaches the final external value (approximately 50 mbar).
A closing device then hermetically seals the cavity so as to enable the pressure to fall inside the container to the final internal value which is lower than the final external value.
Simultaneous pumping from the cavity and from the container can present considerable problems since it is difficult to control the rate at which the pressure falls in each of the two elements.
Indeed, according in particular to the cross sections of the passage for air drawn out of the container and out of the cavity, and the shape of the container, the vacuum forms very rapidly in the container and more slowly in the cavity, so that during a particular length of time, the pressure difference between the interior of the container and the cavity reaches a value greater than the capacity of the container to resist mechanical crushing.
Generally, a PET container does not withstand more than a pressure reduction of 70 to 80 mbar.
The difference in pressure between the two elements therefore causes the container to collapse on itself which causes the container to be rejected and possibly causes the treatment machine to stop.
In order to avoid these problems, it is possible to increase the mechanical strength of the container, for example by increasing the thickness of the walls.
This solution is not satisfactory since it leads to an increase in the weight and cost of the container.
The mechanical strength of the container may also be increased by choosing a suitable shape.
This solution is not satisfactory either since it constitutes a technical constraint which prevents the external shape of the container from being freely chosen.